1. Field of the Invention
The present invention relates to an automatic steering device for a vehicle such as an automobile, and more particularly, to an improvement in the control efficiency of such an automatic steering device.
2. Description of the Prior Art
Various devices have been proposed to automatically steer vehicles such as automobiles. Primarily, the art of automatic steering of vehicles was to automatically drive an automobile along a highway or a street with an assistance of various guide means. In the art of such an automatic automobile driving, the control is generally conducted such that a deviation of an actual driving course from a target driving course is detected and the steering system is adjusted to cancel the deviation according to a feedback control incorporating various combinations of proportioning, integrating and differentiating processes of control parameters. For example, in Japanese Patent Laid-open Publication 1-161408 (1989), an automatic steering device for an unmanned vehicle is so constructed that a deviation from a driving course of the vehicle is detected as a combination of a lateral shifting, a differentiation of the lateral shifting by a drive distance, an integration of the lateral shifting by the drive distance and an angle of posture of the vehicle, so that the detected values are multiplied by respective gains to be fed back for an automatic steering of the vehicle, wherein an integration of the lateral shifting by time and an angular velocity of the vehicle are further fed back with a multiplication of respective gains, for a steering motor being actuated at a speed calculated based upon a difference between a target value and the fed-back value under a condition that the steering angle does not exceed a predetermined allowable limit.
On the other hand, it has also been tried to park or garage an automobile by an automatic steering, as proposed, for example, in Japanese Patent Laid-open Publication 4-55168(1992) or Japanese Patent Laid-open Publication 2000-118334. In Publication 4-55168, there is described an automatic steering device for garaging an automobile according to a predetermined pattern by getting positional information of the vehicle body with reference to a side wall of a garage or by a reflection of a light beam on a mirror plate embedded on a floor at the entrance of the garage, wherein the garaging of the automobile according to the predetermined pattern is fundamentally conducted by steering a pair of steering vehicle wheels according to a predetermined schedule relative to a longitudinal movement of the automobile. In Publication 2000-118334, there is described a parking assisting device adapted to estimate by a micro-computer a steering behavior for parking an automobile and vocally advise the driver of successive operations of a steering wheel.
By incorporating the so-called PID control in the automatic steering, as in the above-mentioned Publication 1-161408, the vehicles will generally be driven to follow a target course by being home biased to the target course when the actual course deviates from the target course, with a home biasing steering force being applied thereto in proportion to the deviation of the actual course from the target course, while such a home biasing steering force is increased more rapidly when the deviation occurs more swiftly, with a long termed adjustment of the control so that no large biasing is caused by an accumulation of a small deviation.
However, in the steering of the vehicle wheels, a larger deviation of an actual course of the rotating vehicle wheels from a target course thereof generates a larger force for more deviating the rotating vehicle wheels from the target course. Therefore, when an automatic steering of the vehicle wheels is controlled by a conventional PID control such that a home biasing force or torque applied to the steering vehicle wheels is increased proportionally to the biasing of an actual course from a target course, even with a forecasting modification by a differentiation of the deviation or a feedback of an accumulation of deviation by an integration, the increased home biasing force or torque is fundamentally balanced by the increased deviation force due to the increased deviation, so that a time required for canceling the deviation is proportionally increased according to the increase of the deviation.
Indeed, however, it is contemplated that the deviation of an actual course of the rotating steering vehicle wheels from a target course thereof should desirably be canceled at a higher speed as it is larger, because a larger deviation of the actual course from the target course is more harmful against the correct course control, while such a larger deviation is less sensitive to a high speed correction control.
Therefore, it is a primary object of the present invention to provide an automatic steering device for a vehicle such as an automobile, by which a pair of steering vehicle wheels are automatically steered so that a deviation of an actual course of rotation of the steering vehicle wheels from a target course thereof is canceled at a higher speed when the deviation is larger at least up to an appropriate threhold value.
According to the present invention, the above-mentioned primary object is accomplished by an automatic steering device for a vehicle having a pair of steering vehicle wheels, a steering wheel, and a power-assisted steering mechanism for converting a steering movement of the steering wheel to a corresponding steering movement of the steering vehicle wheels, comprising:
a first means for generating a target steering movement of the steering vehicle wheels; and
a second means for intervening in the steering mechanism for actuating the steering mechanism so that the steering vehicle wheels are steered to follow the target steering movement in such a feedback manner that a deviation of an actual steering movement from the target steering movement of the steering vehicle wheels is canceled at a speed of cancellation which is increased along with an increase of the deviation at least up to a threshold value determined therefor.
When the automatic steering device is so constructed that, if an actual course of rotation of the steering vehicle wheels deviates more from a target course (within the threshold value), the steering vehicle wheels are steered to cancel the deviation at a higher speed of cancellation, the vehicle is steered to follow the target course at a higher overall precision throughout the time of operation, rendering a higher responsiveness of the control, without sacrificing the stability of control, because the speed of home biasing is increased only when the deviation from the home position is large, while the speed of home biasing is decreased along with approaching to the home position.
In the automatic steering device of the above-mentioned construction, the second means may cancel the deviation by a cyclic repetition of a detection of the deviation and an actuation of the steering mechanism toward the cancellation of the deviation at each speed of cancellation due for each deviation. By such an arrangement, the automatic steering control according to the present invention can be carried out by a step-wise computer control according to a circulation through a series of control calculation or judgment steps.
In the automatic steering device of the above-mentioned construction, the second means may modify the speed of canceling the deviation according to a friction coefficient of a road surface so as to be relatively higher or lower according to whether the friction coefficient is relatively higher or lower, respectively. By such an arrangement, the automatic steering control is more precisely adjusted to the road surface conditions.
In the automatic steering device of the above-mentioned construction, the second means may modify the speed of canceling the deviation according to a vehicle speed so as to be relatively lower or higher according to whether the vehicle speed is relatively higher or lower, respectively. By such an arrangement, the automatic steering control is more appropriately adjusted to the particular steering performance of a rotating wheel dependent on the rotation speed.
In the automatic steering device of the above-mentioned construction, the second means may stop canceling the deviation when a vehicle speed has lowered below a threshold value determined therefor. By such an arrangement, the automatic steering control of the vehicle for parking or garaging the vehicle is carried out at a higher efficiency without causing a useless consumption of a battery or other electric accumulator.
In the automatic steering device of the above-mentioned construction, the second means may stop canceling the deviation when the deviation has lowered below a threshold value determined therefor, while actuating the steering mechanism so that the steering vehicle wheels are held against being moved in a steering direction due to an elastic deformation of tires of the steering vehicle wheels. By such an arrangement, the automatic steering control for parking or garaging the vehicle is carried out at a high stability without causing an undue oscillation in the final stage of control.
In the automatic steering device of the above-mentioned construction, the deviation cancellation speed may be so targeted as to increase along with an increase of the difference up to the threshold value determined therefor and to saturate at a final value for a further increase of the deviation. By such an arrangement, the cancellation speed can be more flexibly controlled at a reasonable cost of the automatic steering device.
In the automatic steering device of the above-mentioned construction, the target deviation cancellation speed may be so modified according to a friction coefficient of a road surface as to be relatively higher or lower against the deviation according to whether the friction coefficient is relatively higher or lower, respectively. By such an arrangement, the cancellation speed can be more flexibly controlled to be more precisely adjusted to the road surface conditions at a reasonable cost of the automatic steering device.
In the automatic steering device of the above-mentioned construction, the target cancellation speed may be so modified according to a vehicle speed as to be relatively lower or higher against the deviation according to whether the vehicle speed is relatively higher or lower, respectively. By such an arrangement, the cancellation speed can be more flexibly controlled to be more appropriately adjusted to the particular steering performance of a rotating wheel dependent on the rotation speed at a reasonable cost of the automatic steering device.
In the automatic steering device of the above-mentioned construction, the second means may stop canceling the deviation when the target deviation canceling speed is not followed by an actual speed of movement of the steering vehicle wheels beyond a threshold difference determined therefor. By such an arrangement, it is avoided that the automatic steering device is unduly operated when a steering vehicle wheel was blocked against a steering by a trimming flange or the like.
The automatic steering device of the above-mentioned construction may further comprise a third means for detecting if the steering wheel is turned by a driver, so as to dispatch an alarm to the driver when the deviation canceling speed is not followed beyond the threshold difference determined therefor with no driver""s turning of the steering wheel. By such an arrangement, the driver is informed that a steering vehicle wheel was blocked against a steering by a trimming flange or the like, discriminating that the steering device is unduly operated by himself/herself.